The present invention relates to a non-invasive device for measuring physiological processes. More particularly, it concerns a device that can be applied externally to the body of an animal or human to detect and quantify displacement, force, motion, vibration and acoustic effects resulting from internal biological functions. Specifically, an inexpensive device is disclosed that is compact, light, portable and comfortable, and operates satisfactorily even with imprecise location on the body, ambient noise, motion and light.
Traditionally, non-invasive gauging of physiological processes in vivo has been accomplished either with large, complex and expensive techniques such as X-ray, tomography, magnetic resonance and ultrasound, all of which demand the skills and infrastructure of a medical institution, or with simpler, more portable equipment. In the latter category, devices employing superficially attached electrophysiological electrodes (electrocardiogram (ekg), electroencephalogram (eeg), electromyogram (emg), electrooculogram (eog)) are prevalent, but require skill and surface preparation for proper electrode attachment and cause skin irritation with prolonged use. Small portable xe2x80x9clightxe2x80x9d devices using optical, ultraviolet and infrared spectroscopic and absorption techniques are also implemented widely based on the availability of miniaturized electromagnetic sources, detectors and support electronics. However, these devices are quite susceptible to interference from movement and/or ingress of ambient light. Motion, displacement, vibration and acoustic sensing techniques have also been developed extensively but have yet to see widespread adoption because their inherent sensitivity to movement, location and noise makes it difficult to interpret signal changes in a normal ambulatory environment. Such techniques range from commercially available, simple, passive actigraphy devices to complex servo-driven systems that react instantaneously to each force or displacement as it is generated by the body. The actigraph is typically a casing attached to the body containing a suspended accelerometer element that responds to all magnitudes and frequencies of motion. During physical activity, it therefore reflects the level of effort being exerted overwhelming any other physiologically derived signals, and during quiet rest periods it becomes sensitive to the body""s internal xe2x80x9cballisticsxe2x80x9d such as breathing, tremor, and heart and arterial pulsation. The servo driven devices are generally invoked to quantify physiological parameters such as blood pressure where the force applied to the actuating element can be used as a measure of the force deriving from the biological function. Typically, these devices are quite bulky, demand external power or compressed air supply and require extensive computation capability. In between these extremes lies a spectrum of devices as described below, covering a range of complexity, accuracy and cost, and varying in effectiveness at combating extraneous signal pick-up. Because of the pre-eminent importance to modern medicine of evaluating cardiovascular function, emphasis has been placed on devices that measure heart rate, arterial pulse profile and blood pressure in the review that follows.
Commercially available devices exist which measure heart rate and provide the output on a wrist-mounted display similar in appearance to a wrist-watch. However, none exist which operate continuously and autonomously without ancillary equipment and without requiring some operation to be performed by the wearer.
1. Chest Band Pick-ups
A large number of products are available commercially from manufacturers like Acumen, Bodyguard, Cardiosport, Cateye, Polar, Performance, Sensor Dynamics, Sigma, Sports Instruments and Vetta, which sense the electrical activity of the heart through electrocardiogram type electrodes mounted in an elasticated chest band. The resultant electrical spikes are then identified and transmitted to a display that may be mounted anywhere within the range of the telemetered signal xe2x80x94in a wrist-watch type devise, a bicycle handlebar mounted readout, or a custom computer display on a piece of exercise equipment. However, even on far forward bicycle handlebars the signal can be lost. A typical device is described in U.S. Pat. No. 5,690,119 to Pekka.
While the devices typically provide steady and reliable readings, they are best suited to use for brief periods during exercise rather than as a continuous long-term or all-day monitor. In addition, the readings are affected by other users nearby utilizing similar heart rate monitors, electromagnetic radiation interference, particularly from power lines and motorized equipment. Accordingly, they typically will not work in an automobile, near TV""s and computers, and on certain types of exercise equipment using electric motors and video screens. While less constraining than an ear clip or finger stall with their associated wires, the chest band takes time to put on under clothing and is a physical encumbrance to the exerciser. In addition, it typically only functions when good electrical contact is established to the skin, either by moistening with water or through sweat during vigorous exercise. During very vigorous activity such as mountain biking the chest-band will often slip down from the optimum pick-up location.
2. Handlebar Pick-ups
Certain types of exercise equipment, such as treadmills, stair-steppers, etc, are outfitted with electrically conducting handlebars that serve as crude electrocardiogram leads. The change in gross position of the cardiac dipole during the course of each beat is manifest as a measurable shift in surface potential between the right and left side of the body and can be picked up at the gripping surface of each hand.
Although this provides feedback while the exercise equipment is being used, all readings are lost when physical contact with the equipment is broken.
3. Ear Lobe Pick-ups
Some exercise equipment and heart rate monitors come with a small infrared transmitter (a light emitting diode is a typical example) and detector that clips onto the ear-lobe and picks up the fluctuation in IR transmission through the ear as the capillaries fill with blood and then drain during the cardiac cycle. The pulse data is hard-wired from the sensor to a display mounted on the wrist or other convenient location. A typical commercially available product of this type is the Cateye PL-6000.
In addition to being uncomfortable, and awkward (because a wire has to be run from the detector on the ear to a visible display), these devices are prone to interference from movement and changing ambient lighting conditions. The slightest head motion causes changes in the amount of IR radiation detected, hence the apparent pulse rate, due primarily to the light that leaks in around the edges of the detector window.
U.S. Pat. No. 5,490,505 to Diab et al. describes such a device, typically configured as a pulsoximeter on the finger or ear with two different wavelength LED""s shining through the tissue to enable an attenuation measurement to be made after propagation through or reflection from the medium. Pulse rate is determined from the periodic attenuation associated with the increase and decrease in arterial flow during a pulse cycle. However the resultant plethysmographic waveform is readily overwhelmed by motion because movement exerts a strong influence on the dynamics of venous blood flow and hence venous blood attenuation of the LED wavelength. Accordingly, the patent describes complex and involved signal processing to recognize the true signal and extract it from exercise-generated noise.
U.S. Pat. No. 4,867,442 to Matthews describes an exercise aid configured as a sweatband around the head with IR or piezoelectric sensors on the ear to pick up pulse. However, the patent provides no enabling disclosure to overcome the motion induced artifacts which would obscure the pulse signal during exercise.
4. Finger Stall and LED Window Pick-ups
While finger stalls are specifically discussed here, the same shortcomings are found with a number of devices used on other parts of the body where there is adequate superficial blood flow, such as the ear, the finger or forehead. Devices requiring fingers or thumbs to be placed over LED windows on the face of a watch are discussed below.
Two devices, the Nissei PU-701 and the Elexis FM-135, are available commercially, which operate on the same principle as the above-described infrared ear-lobe sensors, except that the IR radiation is sent through and received from the fingertip encased in a lightproof finger stall. The resultant information is displayed on a wrist-mounted monitor.
As with the ear-clip, these devices are susceptible to motion and light interference, have external wiring, and are cumbersome and unappealing to wear either during exercise or for routine pulse monitoring during inactive parts of the day. While the lightproof finger stall gives superior results to the LED window types described below, the readings become inaccurate while gripping any object such as a handlebar.
U.S. Pat. No. 4,807,639 to Shimizu describes a watch with either an LED and phototransistor window on its face or a removable finger stall wired to the electronic circuits in the watch case. In either case, a finger is disposed simultaneously over both the LED transmitter and the phototransistor detector (for the finger stall version, both are housed within the stall) such that light reflected by the finger surface, and hence modulated by pulse is detected and counted at the receiving phototransmitter. Because the device only works when the user is motionless and the finger is carefully and properly positioned, the watch is equipped with bar displays corresponding to inadequate readings thereby guiding the user to reposition the finger or move into more favorable ambient lighting conditions.
5. Hand Held Devices
Both ECG type and IR detection type pulse measurements can be accomplished with devices that are held in the hand. One such instrument, the Sportline 390 is held in one hand while the thumb of that hand is placed over an IR window exploiting the same principle as described above for the ear lobe and finger stall sensors. Even though this only occupies one hand, this still constrains most types of activity and the hand must be held very still during a measurement.
Free-standing devices have been developed which operate on the same electrocardiogram principle as the above referenced handlebar pick-ups. In a device made by Pulse Time, the instrument is held in one hand thereby making contact with one electrophysiological electrode and the other hand is brought across to contact an electrically separate part of the device thereby measuring a potential difference across the body during the cardiac cycle. The watches with contact plates described below operate on this same principle. The Biosig Instruments Instapulse series of devices use electrophysiological electrodes embedded in bars and handles that are gripped with both hands. In the Heart Rate 1-2-3E, the thumb of each hand is applied to a different electrode surface on a hand-held unit.
Clearly, these devices are only suitable for intermittent spot checks on pulse because they only work as long as both hands are being used to make the electrical contacts. This is incompatible with exercise and most other types of activity particularly since there is a delay of several seconds after contacting both electrodes before the pulse is displayed.
1. Watches with LED Windows
Watches have been developed and made commercially available by companies such as Casio and Innovative Time, where the pulse is measured by holding a thumb or fingertip over a small LED mounted in a watch face. The IR from the LED penetrates the superficial capillaries of the finger and the amount of IR reflected back to a co-located detector changes as the volume of blood in the capillaries varies during the course of a pulse.
These instruments have all of the disadvantages common to IR based devices as described above for the finger stall and ear lobe types, such as extreme motion sensitivity, and taking several seconds following application of the finger before a pulse reading is registered. They cannot be used during exercise where hands and arms are involved, and are only useful for occasional discontinuous readings because both hands are needed to take a measurement. In addition, the instructions issued with these watches reflect numerous practical difficulties with the measurement because of sensitivity to ambient lighting conditions, sweat and body hair.
2. Watches with Contact Plates
Working on a similar principle to the above described Handlebar Pick-up pulse monitors, watches have been developed by Seiko, Advanced Body Metrics and Physi Cal Enterprises, for example, which have a small metal plate on the watch face insulated from the metal backing of the watch which rests against the wrist. The user brings across a finger from the opposite hand to touch the insulated metal plate thus setting up the usual two electrode electrocardiogram scheme with one contact as the fingertip and the other, the back of the wrist of the opposite arm.
As with the LED window watch, the device is impractical for continuous reading or measurement during exercise because both hands and arms are tied up by the measurement. In addition, there are electrical contact problems and hence readings become unreliable when the user""s wrist under the metal backing of the watch is hairy.
A device of this type is described in U.S. Pat. No. 4,938,228 to Richter where, in the preferred embodiment, the biosignal reflecting the pulse is generated by an electrocardiogram electrode on one wrist and another electrode on the finger of the opposite hand. Other proposed locations included any two well separated parts of the body of opposite polarity. Alternatives to the ECG electrode are microphonic, piezoelectric, photo-optical and capacitative types, but there is no enabling disclosure describing their implementation. In a non-clinical environment, the signal is confounded by movement, low signal to noise in certain individuals, and poor electrical contact to the skin. The patent therefore describes very complex and computationally intensive signal processing and pattern recognition schemes (which incorporate a time-consuming learning cycle before any pulse data can be registered) to overcome these extreme difficulties encountered in trying to extract clean pulse data from non-medical, non-chest mounted ECG leads. After applying the computational methods of the invention, 50% of subjects still gave unsatisfactory readings during exercise, suggesting the need to slow or stop the exercise or abrade or wet the skin.
The above-described shortcomings of wrist pulse displays which require ancillary equipment or which require manual intervention, have spawned a large number of attempts to develop a device that is continuous reading, autonomous and fully self-contained on the wrist. While many have been invented and patented, none have been successfully developed into commercially viable products. Some of the more promising attempts and their shortcomings are reviewed below.
1. Pressure/Displacement Sensors
In U.S. Pat. No. 3,807,388 to Orr, a heartbeat rate monitor or personal pulse indicator is described employing a sensor that is either a xe2x80x9cpressure sensitive resistorxe2x80x9d or a xe2x80x9cpressure sensitive transistorxe2x80x9d. Beyond these generic terms, no further description is provided for implementing these sensor types to ensure high signal to noise, to avoid signal loss with wrist movement or to adapt to a wide range of input signal strengths. For either sensor type, the sensing element is shown embedded in, and protruding from, the inner surface of a wrist strap to react to pressure changes at the surface of the wrist during the course of a pulse. To counter random movement of the strap, the sensor is held firmly in one position on the wrist and not allowed to slide by virtue of a stabilizing frame that also protrudes from the strap beyond the circumference of the sensor element. The sensor elements shown to be very small in comparison to the width of the strap, and from the figures, measures approximately {fraction (1/10)} inch in diameter.
In U.S. Pat. No. 4,120,296 to Printz, a pulsimeter is described for indicating the average heart beat rate using a sensor adapted for flat application against the body, preferably at the wrist. The device merely uses a commercially available sensor, known as a Hewlett Packard digital plethysmograph model 14301A. From the drawings and method of application, the sensor appears to be about xe2x85x9cxe2x80x3 in diameter and thin enough to completely embed in a conventional watch-band ( less than {fraction (1/10)}xe2x80x3). There is no discussion of modification to the sensor or its implementation to improve signal to noise ratio, compensate for location specificity or to accommodate a wide range of input signal strengths.
In U.S. Pat. No. 4,009,708 to Fay, a pulse recorder is described, self-contained in a wristwatch type case. The sensor is either a microphone or a pressure sensor, approximately xc2xc-xe2x85x9cxe2x80x3 in diameter as shown in the figures, but no further description or implementation is provided to explain how practical issues are resolved, such as location sensitivity, signal strength changes and competition with noise signals. Moreover, the sensor forms part of the back face of the case and therefore bears on the upper surface of the wrist where there is very little acoustic or pressure activity occasioned by the pulse.
In U.S. Pat. No. 3,742,937 to Manuel, a self-contained, compact cardiac monitor is described for strapping on the wrist over the region where the radial pulse is normally detected. Two sensor mechanisms are described, both using a differential pressure detector scheme within a chamber of encapsulated low viscosity oil. The outer surface of the sensor is constructed of pliable silicone rubber which moves in response to the pulse and thereby displaces silicone oil in a centrally located pocket. In one case, a thin leaky silicon diaphragm with embedded strain gauges divides the encapsulated oil into two chambers. During the course of a pulse, the oil moves and deflects the diaphragm producing a measurable signal on the strain gauges. In the other case, the oil is divided into two sub-chambers by the flexible central conductor plate of a capacitor so that the capacitance of the circuit changes in response to oil movement during a heart beat. Overall dimensions of the silicone rubber enclosure are xc2xc-xc2xdxe2x80x3 in diameter and 0.1-0.2xe2x80x3 thick.
In U.S. Pat. No. 4,281,663 to Pringle, a physical fitness indicator is described utilizing a piezoelectric displacement sensor similar to a crystal phonograph pick-up attached to the wrist, throat or ear lobe. Although Pringle states that it is technologically feasible to house the whole device in a watch-type case with the sensor bearing on the radial pulse, no description of the sensor is provided, nor is there any discussion of how to overcome location sensitivity and motion artifacts. In addition, while the signal conditioning electronics and processing software identify and count pulses, no consideration is given to managing input signals with vastly differing magnitudes and signal to noise ratios.
In U.S. Pat. Nos. 4,331,154 to Broadwater and 4,307,727 to Haynes, a self contained wrist-mounted blood pressure monitoring system is described employing a very small diameter rod digging into the skin above the radial artery. The rod has a tiny piezoelectric crystal mounted on the end that engages with the skin to respond to the displacement pulse as a bolus of blood passes down the vessel. No attempt has been made to match the impedance of the hard ceramic crystal on top of the hard rod to the mechanical impedance of the physiology. A simple tensioning mechanism adjusts the length of the wrist band until the artery is flattened to half its initial diameter. In fact, it is likely that most of the motion produced by pulsation in the artery would be taken up in compliance of the band and mechanisms behind the piezoelectric crystal rather than getting coupled into the crystal itself.
2. Ultrasonic Devices
In U.S. Pat. No. 4,086,916, a cardiac monitor wristwatch is described with an ultrasonic transmitter and receiver in the wristband. The device registers movement of the outer surface of the radial artery by frequency difference between the impinging and reflected signals as the artery expands and contracts during a heartbeat. While the ultrasonic transmitter is power consuming and would therefore not appear suitable for continuous monitoring with a typical small watch battery power supply, it is claimed that the existing watch battery will suffice because the transducers are very directional and therefore efficient.
While numerous methods have been proposed for continuously and non-invasively measuring blood pressure as an alternative to the invasive arterial line, none have yet reached the point of widespread medical acceptance and commercial availability.
In the standard arterial line technique, a pressure sensor is inserted into the artery or into a contained volume of blood in free communication with the artery, and generates a signal that continuously and accurately reflects the blood pressure. However, the procedure is uncomfortable, expensive, consumes care-provider time and risks embolization, nerve damage, infection, bleeding and vessel wall damage.
The standard non-invasive device is the automatically inflating pressure cuff (applied over either the brachial or the radial artery), which can be uncomfortable with repeated use and only provides discontinuous sample readings at the time the device activates. The device typically requires 15-45 seconds to take a reading and, because it is occlusive, a minimum of 15 seconds must be allowed between readings to allow sufficient venous recovery. Also, frequent cuff inflations can produce ecchymosis and nerve damage under the cuff after extended periods of use. The oscillometric signal utilized by a cuff is also very sensitive to motion of the arm.
Another commercially available non-invasive blood pressure measuring device is the Finapres which employs photoplethysmography, e.g., as disclosed in U.S. Pat. No. 4,846,189 to Sun, U.S. Pat. No. 4,869,261 to Penaz and U.S. Pat. No. 4,406,289 to Wesseling, to interrogate the diameter of a blood vessel and correlate this to pressure. A small cuff containing an IR source and detector tuned to the wavelength for hemoglobin is inflated around a patient""s digit. After measuring the mean arterial pressure, from then on, the device adjusts the applied pressure to maintain the diameter of the artery constant (transmural pressure held at zero). As the patient""s blood pressure changes, this is then reflected in a change in the pressure that must be applied to hold the diameter of the artery fixed. This device has been found clinically unsatisfactory because changes in arterial diameter due to changes in arterial wall compliance (vasomotor tone) are falsely registered as blood pressure changes. In addition, the constant pressure is uncomfortable over extended periods of monitoring, and the peripheral blood pressure measured in a finger is not necessarily representative of central blood pressure, particularly if the vascular circulation is poor or there is peripheral vasoconstriction.
In U.S. Pat. No. 5,406,952 to Barnes, a wrist mounted device is disclosed for non-invasively measuring blood pressure, comprising a pressure/displacement pick-up with a piezoelectric sensing element mounted over the radial artery on the underside of the wrist. The piezoelectric element is a unimorph disk constructed from a 10 mil brass substrate with a 10 mil layer of piezoelectric ceramic deposited on its surface. The element is activated by a small diameter rod protruding from the device into the skin above the radial artery at one end and bearing on the center of the disk at its other end. The disk is suspended from a compliant circumferential ring of silicone rubber which diminishes the amount of energy transferred into the electroactive element. In principle, as a bolus of blood passes along the radial artery during a pulse, the artery expands, raising the skin and flesh above it and displacing the rod into the piezoelectric element. In practice, it would be very difficult to assure accurate and reliable positioning of the small diameter rod over the radial artery, particularly in view of lateral displacement of the radial artery as the wrist is rotated and repositioned. Accordingly, there would be an unacceptable amount of signal attenuation and even complete drop-out accompanying normal movement. To counteract this position-dependent effect and prevent motion artifact as the rod gets pulled against the skin, patentee uses an adhesive foam interface to hold the actuating rod exactly in place. However, the band used to keep the device on the wrist does not run over the sensor housing and therefore does not apply pressure to help keep the housing in mechanical contact with the wrist. In addition, the signal levels from this device would be extremely low and hence difficult to extract from noise because the unimorph disk configuration is relatively stiff and very little force is applied to it by the rod because of the small area of contact between the externally protruding end of the rod and the artery wall which provides the driving force. Another way of considering this is that the actuating rod is fairly difficult to move into and out of the plane of the disk and therefore is effective at resisting the motion of the artery wall. Accordingly, the flesh and skin expansion during each pulse will tend to xe2x80x9cside-stepxe2x80x9d the rod and instead expand into the neighboring tissue. To derive a blood pressure from the displacement signal, patentee uses a signal processing scheme that is totally independent of signal amplitude and instead relies on an empirically derived set of correlation factors applied to their pattern recognition and morphology analysis.
In U.S. Pat. No. 5,485,848 to Jackson, a non-invasive, non-intrusive, convenient and portable blood pressure measuring device is described based on sensing the user""s arterial wall movement. Although no details or implementation are provided, it is suggested that the sensing can be accomplished with an aneroid chamber, strain gage, optical motion sensor or hydraulic sensor. Absent such details, it is not clear how the devices would be applied to overcome signal to noise problems, widely disparate input signal levels and signal attenuation with position changes. The one sensor implementation that is described, utilizes a film of piezoelectric polymer embedded in the inner surface of a wrist band. Much attention is given to band tensioning and locking mechanisms in addition to automatically controlled motorized tension adjusters. Even with these measures, it likely that the signal to noise ratio would be inadequate for pulse measurement with this device, particularly as the signal became obscured by movement and other extraneous phenomena.
In U.S. Pat. Nos. 4,960,128 and 5,163,438 to Gordon, a film of piezoelectric Kynar is disposed over the artery to sense the pulse and transmit the signal over a 10 foot cable to a signal processing means. Because the film has inherently poor electromechanical coupling and is applied directly to the skin without any modification to match its mechanical impedance to the physiology under test, the example pulse trace is very noisy and must be integrated to smooth the curve and eliminate noise. Blood pressure is obtained by feeding the Kynar signal into a standard arterial line monitor after applying a correction factor to the signal amplitude. The correction factor is an empirical scaling value that depends on how much of a frequency shift there is between the frequency corresponding to the maximum amplitude (based on a Fourier transform) of the pulse in question and the initial calibration pulse. Signal processing is applied. There is no provision for motion or other noise compensation, or means to handle a wide dynamic range of input signal amplitudes.
In PCT Pat. No. WO 95/18564 to Kaspari, a piezoelectic film is disposed over the artery in question together with two outside elements whose output is summed and subtracted from the main sensor to minimize sensitivity to motion artifact. Blood pressure is determined by breaking down the measured pulse profile into all of its components (both phase and amplitude) related to different physiological functions and identifying those that correlate best with changes in blood pressure. Pattern recognition and neural networks are used to xe2x80x9clearnxe2x80x9d which features provide the best match to blood pressure during a xe2x80x9ctrainingxe2x80x9d phase on the patient. These features and empirical relationships are constantly updated based on calibration values from a non-invasive cuff. To shorten the training process, a historical database is developed on a large population of patients where blood pressure is measured continuously and invasively via catheter and compared side by side with the non-invasive piezoelectric signal to predetermine which features correspond best with blood pressure.
In U.S. Pat. No. 3,926,179 to Petzke, a tonometer is described comprising a bulky apparatus resembling a long thick rod applied orthogonal to the wrist above the radial artery. The rod is not suitable to be self-contained on the wrist because it incorporates a power-consuming servo-controlled pressure applying means to compress the artery to half its thickness and thereby maximize the signal strength and eliminate artifacts due to circumferential components of the arterial expansion. The sensor mechanism positioned at the center of the pressure applying piston is smaller in diameter than the artery under interrogation and comprises a flexible resilient diaphragm with a few tiny elements on its surface that expand and contract as the diaphragm flexes in response to the pulse. Since the electrical resistance of the elements changes as they are mechanically strained, a differential resistance between centrally and peripherally disposed elements provides a measure of the pulse amplitude. Because the sensor is so small, it would have to be very accurately placed and held in position, and the signal strength would be very susceptible to arm movement or rotation. No provision is made for rejecting motion artifacts.
Other tonometric devices are described in U.S. Pat. Nos. 4,269,193, 4,799,491 and 4,802,488 to Eckerle, U.S. Pat. No. 4,423,738 to Newgard, U.S. Pat. No. 5,033,471 to Yokoe, and U.S. Pat. No. 5,165,416 to Shinoda, and a commercial implementation is available through Colin Medical. This device uses a multielement piezoresistive sensor to pick up the amplitude of the pulse at the wrist and use this to follow blood pressure trends between oscillometric cuff readings. The sensing element is small compared to the artery and so will give readings that are very location dependent as the patient moves.
Another non-invasive technique uses pulse transit time (typically to the ear lobe and to the finger) to correlate with blood pressure, as disclosed in European Patent 0 443 267 A1 to Smith. The arrival time of the pulse at the ear and finger is measured by photoplethysmography at each location and used to calculate a change from the calibration pressure measured with a cuff. However, the technique does not compensate for changes in shape of waveform, and even small movements and noise cause relatively large errors in the delay time because the transit times are very short along major arteries.
PCT WO 95/28126 to Caro describes an active interrogation scheme for determining blood pressure by measurement of pulse wave velocity down the arm, whereby a small amplitude excitation signal (20-600 Hz) is superimposed onto the normal pulse profile using air pressure modulation in a cuff inflated on the forearm near the elbow. The combined pulse and superposed perturbation are received by a piezoelectric film over the radial artery and the resultant signal processed by analysis of phase relationships to separate out the three constituentsxe2x80x94the pulse, the perturbation and noise. Pattern recognition is used to ensure that a true pulse has been received. Blood pressure is determined from look-up tables based on both the velocity and phase of the superposed exciter wave. Values in the tables derive from statistical measurements taken on a representative population of patients. The piezoelectric receiving sensor is held in place with a second pneumatic cuff that also acts as a baffle to keep out external noise. A third pneumatic cuff is applied over the biceps in conventional fashion to measure a calibrating oscillometric blood pressure. In one alternative embodiment, the need for a calibration cuff is eliminated by including a second excitation signal on the pulse carrier wave. In another, the exciter and sensor are both located over the wrist. However, the device is not suitable to be self-contained on the wrist because of the computationally intensive signal processing and the need for bulky and power consuming inflation cuffs and regulators.
In U.S. Pat. No. 4,807,638, electrical bioimpedance measurements are made between two segments of body tissue to determine the pulse wave velocity.
Many previously described attempts to gage pulse and blood pressure rely on direct contact between a sensing element and the skin above an artery. While new classes of extremely pressure sensitive materials and devices are now available in forms that can readily be miniaturized to the configuration of an unobtrusive wrist sensor, none of these can meet the very exacting requirements of a wrist-mounted pulse sensor without significant modification, and incorporation into systems which accommodate the physiological variables and the motion and noise environment. Examples of these materials and devices include the elastomeric sheet material polyvinylidene difluoride (PVDF), semiconductor strain gages, pressure sensitive resistors and diodes (and their semiconducting counterparts), and piezoelectric crystals and composites. Many attempts have been made to sense pulse by interfacing these materials and devices directly to the body at locations such as the neck, forehead, wrist and thigh where arteries lie close to the surface of the skin. The following patents provide a good range of examples where the material or device is placed directly against the body at the point above the artery, either alone or as the outer surface of a contact pad in a more elaborate application mechanismxe2x80x94U.S. Pat. Nos. 4,784,152; 4,901,733; 4,947,855; 5,101,829; 5,131,400; 5,238,000; 5,439,002; 5,467,771; 5,497,779; 5,551,437; 5,515,858; and 5,509,423. In all cases, even though the sensing element is very sensitive, the pulse pressure wave manifest at the body surface is so subtle that the target signal is readily lost in or confused with noise and other environmental influences.
While some inventors have recognized the benefit of capturing arterial displacement signals over a broad area on the surface of the body and have provided relatively large contact pads to accomplish this end, these pads usually contain sensing elements in the contacting surface and therefore do not work satisfactorily for the reasons given in the preceding paragraph. However, the present invention preferably employs an inert (non-sensing) contact pad that acts through a mechanical load capture and transfer element which in turn activates the sensitive material or device. The inert pad responds to a displacement at any point on its surface and focuses the energy onto the sweet spot of the sensor. The prior art techniques introduce complexity into the system by attempting to achieve wide area pick-up by populating the contacting surface area of the pad itself with actual sensing elements. Any energy incident on the contacting surface area at a point not populated by a sensor is not detected by the device. An additional problem, as will be described below, is that this direct interfacing technique does not provide enough separation of the signal from the noise. In addition, new layers of computation and optimization are required to continually select the sensor with the best signal and/or weight inputs from the different sensors.
To avoid the above referenced problem of low absolute sensitivity to the pulse wave and high susceptibility to noise with techniques and devices that use direct contact sensing, techniques have been proposed that separate the two functions of contacting the body surface and sensing the displacement. For example, in U.S. Pat. No. 3,838,684, surface contact is effected with a fluid filled elastomeric bladder while the arterial motion signal is picked up remotely in the fluid through distortion of a central baffle plate which in turn actuates a capacitative displacement sensor or strain gages located on the plate. In U.S. Pat. Nos. 4,058,118, 4,409,983 and 4,561,447, the pulse is initially interfaced with a contact pad which then transmits the load to the center of a small beam with a strain gage or a beam of piezoelectric material, either crystalline, bimorphic or configured as a bender bar. While this arrangement is used in 4,058,118 to transfer pressure received over a gently contoured area of contact pad onto the center of a piezoelectric element, the pad is constrained to move only along the axis perpendicular to the plane of the piezoelectric crystal because it can only move within the fixed geometry of a piston and cylinder type arrangement. Automatic gain control circuiting on the signal input is employed, but the patent does not address signal susceptibility to mispositioning and motion. Accordingly, it is recognized that the device alone cannot combat the problem of noise and motion infiltration hence requiring that the device only be used during very quiet, stationary periods.
In U.S. Pat. Nos. 4,409,983 and 4,561,447, the same basic mechanism is used as above in U.S. Pat. No. 4,058,118, but soft cushioning material or springs are incorporated into the outer mounts for the piezoelectric or strain gage beam to dampen the structure and mechanically filter out high frequency noise. While patentees claim the resultant motion and noise pick-up are significantly diminished, the soft mounting feature greatly reduces the absolute sensitivity of the piezoelectric beam sensor. Because there is such a huge variation in arterial pulse signal strength, associated with individual differences, changes during the day, changes with limb position, changes with health and exercise, etc., the greatest possible absolute sensitivity of the sensing element is required to ensure that the entire dynamic range can be covered.
In Pat. No. 4,561,447, a single element sensor embodiment of the invention is described which is designed to be hand-held against the neck of a patient by a caregiver who can properly place the device over the carotid artery by lining up the cut-outs in either end of the sensor. Exact positioning is critical because of the particular actuating mechanism, where a presser portion is used to contact the body over an artery and an abutment portion transfers the load onto the center of a flexible beam incorporating a strain gage, mounted at its ends. The size of the presser portion is less than the diameter of the artery under interrogation since the presser portion fits within end slots (cut-outs) in the housing designed to accommodate the projection of the artery at the surface of the body. In addition, the direct spatial relationship of the presser and abutment creates inherent sensitivity to getting mis-positioned because it ensures that only those forces or displacements impinging directly on the relatively small area of presser surface are transferred to the measuring strain gage. This point is emphasized in the disclosure of two multi-element embodiments of the invention developed for the purpose of overcoming distortion associated with the sensor getting displaced in any way from its optimal position on the body. In one of these embodiments, the line of elements traverses the artery, thereby allowing one of the plural presser portions to be located within an area of the neck skin in which the pulse wave of the carotid artery can be detected as a sufficiently high level of electric signals without a distortion. In the other embodiment, the plural sensors allow detection of a pulse wave signal without a distortion which could be caused by misalignment of a detector or sensing device relative to the carotid artery when the detector has a single sensing unit.
The 4,561,447 patent also places great weight on high fidelity replication of the exact pulse profile and defines a distortion parameter that has to be kept below a certain level to satisfactorily meet the requirements of the invention. The principal factor controlling distortion is the depression pressure experienced at the actual sensing surface against the body. Accordingly, a highly compliant (elastic modulus as low as practicable) mounting material (foam, elastomer, springs, etc.) is introduced under the end-mounts of the sensing beam to ensure that the depression pressure remains within the optimal range while the actual application force from the user""s hand varies widely from very light to very strong application. When a high modulus mounting material is used, the depression pressure becomes too high, causing the pulse wave signal to be distorted.
The present invention has been developed in view of the foregoing, and to address the deficiencies of the prior art.
The invention described below is intended to provide a simple, inexpensive, compact and portable device for sensing physiological signals while overcoming the prior art difficulties of susceptibility to noise, motion, orientation (attitude of body member to which the sensor is attached) and inadequate dynamic range to cover the broad spectrum of actual input signals encountered in physiological measurements. To reduce the response of the device to force and acceleration associated with gross body movement, the sensing element, coupling mechanism, and its mounting structure are made extremely light and are all tightly coupled to the body surface at the point of attachment such that both the element and its mount receive the external motion equally and can therefore cancel it as a common mode in a difference measurement. Depending on the demands of the specific application, noise and motion effects are optionally further reduced through subtraction and filtering schemes based on the frequency content detected in a separate optional motion sensitive element or elements, common mode rejection employing another comparable optional sensor, or sensors, located away from the maximum physiological signal, or signal processing to extract recognizable physiological signal patterns from noise and motion artifact. Advanced detection and tracking electronics are used to span the very demanding dynamic range of sensitivity needed to follow signals as they vary widely from individual to individual, and within an individual with level of physical activity, time of day, state of health, limb orientation, etc. The dynamic range is also maximized through use of an extremely sensitive sensing member, firmly mounted with respect to the sensor housing and actuated with considerable mechanical advantage by the physiological process at a very sensitive point on the sensing member. Mechanical advantage is achieved by not placing the sensing member in the contact face of the device so that it can be actuated by a pressure amplification mechanism. Problems with loss, or decreased amplitude, of signal with misalignment from the ideal sensing location are minimized with a novel design of xe2x80x9cfree-floatingxe2x80x9d contact pad that interfaces to the body over an area that is broad enough to encompass most of the area of the body where effects of the physiological process in question are manifest. This interface not only conforms comfortably to the contour of the skin but responds in an essentially omnidirectional fashion to forces and displacements incident at any angle onto the outer face of the pad. While the pad is simply an inert mechanical collector of force and displacement, it couples via a mechanical transfer mechanism to a highly sensitive portion of a sensing member. Optionally, to ensure that device responds to the physiological process but not to unrelated noise or motion, the mechanical transfer mechanism is constrained to respond only in one axis directed towards the source of the physiological phenomenon and thereby reject any motions or forces the contact pad has picked-up in orientations away from this axis.
A specific use of the invention is as a non-invasive device for use on mammals including humans and other animals to sense blood vessel wall motion and blood vessel wall position, measure pulse rate and/or amplitude and shape of arterial pulse profile from which to derive blood pressure. The sensor and conditioning electronics of the device are also suitable for measuring parameters such as vascular wall compliance, ventricular contractions, vascular resistance, fluid volume, cardiac output, myocardial contractility and other related parameters.
To be configured for continuous use as a cardiovascular monitor, the sensor assembly is preferably made as light and compact as possible and applied with a band over a suitable artery on the body. Preferably, the device is made fully self-sufficient with sensor assembly, support electronics, power supply, processing and display all incorporated on the same band.
In a preferred embodiment, the device is non-encumbering and suitable to be worn continuously and to calculate and readout pulse and blood pressure continuously or as required. The device is sensitive to pressure/displacement signals of widely varying magnitude collected over a broad area on the underside of the wrist, and includes signal conditioning electronics that automatically normalize the signal level to facilitate digital processing and counting. Optional data storage and digital interfaces, such as a standard RS232 port or infrared communications link, may also be included to increase the versatility, appeal and usefulness of the device. For this embodiment as a wrist worn pulse counter, it is an aspect of the present invention to provide the following combination of features: no ancillary equipment (such as chest band, ear lobe clip, finger stall, equipment to hold, window or plate to touch) beyond the self-contained wrist device itself; convenient all-day comfort and portability, wearability; continuous ongoing measurement and display; no action or intervention required of the user to activate readings; insensitive to hair, sunlight, sweat; and cleanest possible sensor signal with highest fidelity replication of the motion of the arterial wall movement to minimize or eliminate the need for complex signal processing techniques, learning and pattern recognition.
These and other aspects of the invention will be more apparent from the following description.